1. Field of the Invention
This invention relates to systems and methods for gravel packing a well.
2. Related Art
In the production of hydrocarbons (e.g., oil) from hydrocarbon-bearing formations, a well is drilled from the surface of the earth into the formation. The well may be completed by employing conventional completion practices. For example, casing may be run in the well and cemented, and perforations may be formed through the casing and the cement that surrounds it. This results in an open production interval through which hydrocarbons can flow into the well.
If the production interval is in an unconsolidated or poorly consolidated formation, sand may be produced along with the hydrocarbons. This is undesirable for many reasons. The sand is abrasive and increases wear on components within the well, such as tubing, pumps and valves. The sand must also be removed from the hydrocarbons at the surface. The sand may also partially or completely clog the well, thereby making it necessary to work over the well (which is very expensive). Still further, sand which flows out of the formation may leave a cavity in the formation which may make it unstable and vulnerable to collapse of the formation and the casing.
One means for resolving these problems is to pack the production interval (or at least a part of it) with gravel. (The size and material of the gravel particles may vary, depending upon the particular situation.) The gravel pack serves several purposes. For example, it serves to filter sand from the hydrocarbons that flow into the well. The gravel pack also serves to prevent sand from flowing out of the formation and leaving it unstable. The gravel pack also provides support for the casing and formation in the packed interval so tha they are less likely to collapse.
Conventional gravel packing techniques generally involve the insertion of a well screen into the well. An annulus is thereby formed between the screen and the wall of the well. A slurry of gravel (gravel suspended in a fluid) is injected into the annulus until the volume between the screen and well bore (the wall of the well) is filled.
Conventional gravel packing techniques, however, are not without problems themselves. For instance, it is not uncommon for a gravel pack to have voids (in which the gravel has not completely filled the space). It is particularly difficult to ensure that there are no voids when the interval to be packed is inclined or horizontal. The voids may allow sand to flow into the well and reduce the overall effectiveness of the gravel pack.
One of the primary causes of voids in a gravel pack is the formation of gravel bridges. Gravel bridges form when particles of gravel become lodged between the well screen and wellbore prior to reaching the end of the volume being packed. After a bridge has begun to form, additional particles accumulate as they become lodged between the bridge itself and the screen or wellbore. The lateral extent of the bridge may increase until it eventually blocks further flow of the gravel slurry so that voids form behind the bridges.
One means for resolving the problem of gravel bridge formation is the use of a perforated shroud around the well screen. The shroud effectively separates the annular region between the screen and well bore into an inner annulus between the screen and the shroud, and an outer annulus between the shroud and the wellbore. The shroud does not prevent the formation or growth of gravel bridges, but if gravel bridges form in the outer annulus, the gravel slurry can flow into the inner annulus through the perforations in the shroud. The slurry can then bypass the bridges and flow back into the outer annulus through the perforations. Voids behind bridges can therefore be filled so that a complete gravel pack is formed.
While the invention is subject to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and the accompanying detailed description. It should be understood, however, that the drawings and detailed description are not intended to limit the invention to the particular embodiment which is described. This disclosure is instead intended to cover all modifications, equivalents and alternatives falling within the scope of the present invention as defined by the appended claims.